Sometimes when you try to start an Office app, you might get this error. Follow these steps to fix the problem.
Restart your computer.
Sign in and restart your application.
In most cases these steps solves the issue.
But if it is still their you can try repairing the app.
To Repair the app in Windows 10
Right click on Start button.
Click on Control Panel.
From category view
under the Programs
Select Uninstall a program.
Right click on the office Product
Select How would you like to repair your Office Programs screen.
Then select Online Repair to make sure everything gets fixed
Then select Repair. (Quick Repair runs faster but only detects and then replaces corrupted files)
Online Repair does everything.
After these follow the instruction to finish
Source : Microsoft